The present invention relates generally to methods for heat treating metals and alloys, and more particularly to a heat treat method for making fully lamellar, moderate alpha (.alpha.) grain size microstructures in near-gamma (.gamma.) titanium aluminide alloys having good room temperature strength, ductility, and fracture toughness and high temperature creep resistance.
Near-.gamma. titanium aluminide alloys are extremely useful for applications requiring high temperature strength and creep resistance. In many applications, however, such as in jet engines, the alloys must also exhibit an acceptable level of room temperature ductility and fracture toughness for product manufacture and assembly and for damage tolerance during service.
Although a balance of room and high temperature properties in near-.gamma. titanium aluminide alloys might be achieved in a fully lamellar alloy having moderate a grain size (100 to 200 .mu.m), conventional methods for heat treatment of the alloy have not been successful because .alpha. grain growth in the alloy is rapid and leads to grain sizes of the order of 500 to 1000 .mu.m.
Alternative processing methods which have been developed to overcome the difficulty associated with rapid .alpha. grain growth include the addition of tungsten, titanium diboride particulate, or other alloying material to restrain a grain growth during heat treatment in the single phase .alpha. field. In another alternate method, a desirable balance of room and high temperature properties in near-.gamma. titanium aluminide alloys is achieved wherein canned material is preheated below the .alpha. transus temperature (temperature at which .alpha.+.gamma..fwdarw..alpha. in the alloy) and then extruded. During extrusion, deformation work produces a transient which raises the workpiece temperature into the single phase .alpha. field thus producing a recrystallized, moderate .alpha. grain size. This type of process may be limited to large cross section, bulk formed semifinished parts because of the difficulty of controlling the unavoidable die chilling that occurs during such conventional metal working processes.
The present invention is applicable to substantially any near-.gamma. alloy irrespective of chemical composition, and is well suited to the production of a wide variety of semifinished or finished shapes of thin (.ltoreq.20 mm) cross-section such as flat or formed sheet produced by superplastic forming, thin cross section bar or wire products, or other thin cross section product shapes. Total heat treatment time (typically 10-20 min) is much shorter than conventional heat treatments and does not rely on special alloying additions or on second-phase dispersions to control grain size.
It is therefore a principal object of the invention to provide a method for heat treating metals and alloys.
It is another object of the invention to provide a heat treat method for titanium aluminide alloys.
It is yet another object of the invention to provide a method for producing fully lamellar moderate .alpha. grain size microstructures in near-.gamma. titanium aluminide alloys having good room temperature strength, ductility, and toughness and high temperature creep resistance.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.